The present invention relates to a method of producing dichloroethane (ethylene dichloride) (hereinafter referred to as "EDC") by reacting ethylene and chlorine in a liquid phase at a temperature not lower than 83.degree. C. More particularly, the invention relates to a method of producing EDC wherein the heat of reaction is recovered for efficient utilization.
EDC is industrially important as a starting material in the production of vinyl chloride monomer. Since disclosure in U.S. Pat. No. 2,929,852, a method of reacting ethylene with chlorine at a temperature not lower than 83.degree. C. by feeding the reactants into a liquid reaction medium composed mainly of EDC is known as the so-called high temperature method. The method is advantageous as compared with the low-temperature method in that the heat of reaction can be utilized more efficiently. In accordance with the method proposed in the above U.S. patent, a reaction-distillation scheme is adopted, namely the liquid reaction medium is vaporized by the heat of reaction and the vapor generated is introduced for purification into a distillation column connected with the upper part of the reactor, so that the heat of reaction can be efficiently utilized as a source of energy required for purification of EDC.
In British Patent No. 1,231,127, it is proposed that the heat of reaction should be efficiently utilized by purifying, according to the above reaction-distillation technique, not only EDC produced by the above reaction but also EDC obtained from the oxychlorination step in a vinyl chloride monomer production plant and the unreacted EDC obtained from the cracking step. In U.S. Pat. No. 4,172,099, an improvement is proposed which comprises supplying the EDC from the oxychlorination step to the above-mentioned reaction-distillation step after washing, neutralization, dehydration, low-boiling fraction elimination and like steps and, for the EDC from the cracking step, supplying it to the reaction-distillation step after passing through a step of chlorination. In Japanese patent Publication Kokai No. 90206/78, an improved method is proposed which comprises supplying the EDC recovered from the cracking step to the above reaction-distillation step after passing through a step of chlorination and then a step of removing high-boiling chlorinated components by distillation.
The heat of reaction in the production of EDC by reacting ethylene with chlorine is about 50 kcal/mol. This is about 7 times the quantity of heat required for vaporizing the EDC produced in this reaction. Therefore, when the reaction-distillation is conducted using this heat, a sufficient quantity of vapor can be produced for purifying the EDC from the oxychlorination step and the unreacted EDC from the cracking step as well as the EDC formed from ethylene and chlorine. However, as pointed out in Japanese Patent Publication Kokai No. 90206/78, if the unreacted EDC from the cracking step is fed to the reaction-distillation step, chloroprene and chlorinated derivatives thereof contained in the unreacted EDC exert an adverse influence on the reaction to decrease the selectivity of reaction and, as a result, the yield is markedly decreased. Feeding of the EDC from the oxychlorination step, which contains impurities such as water, ethylene chlorohydrin and chloral, to the reaction-distillation step is also unfavorable since such impurities cause significant corrosion of materials of construction and adversely influence the selectivity of reaction.
For the reasons mentioned above, only the EDC formed from ethylene and chlorine is applicable to the reaction-distillation technique for purification by distillation. For the removal of impurities in the unreacted EDC or the EDC from the oxychlorination step, some other energy source is required, as explained in Japanese Patent Publication Kokai No. 90206/78 or U.S. Pat. No. 4,172,099. Thus, in the prior art, the heat of reaction is not utilized efficiently and energy saving is insufficient.
Another drawback of the conventional reaction-distillation technique is that the content of low-boiling impurities in the purified EDC is high as compared with the case where low-boiling fraction removal and high-boiling fraction removal are conducted in order. In the process comprising taking out high-boiling impurities from the column bottom, low-boiling impurities from the column top and the purified EDC from the middle of the column, the content, in the purified EDC, of low-boiling impurities such as ethyl chloride is significant and these impurities exert an unfavorable influence on the cracking step for vinyl chloride monomer production.
A further drawback is that, in the reaction-distillation method, high-boiling impurities are concentrated in the reactor and cause a boiling point elevation, hence a rise in the reaction temperature, so that it is difficult to maintain the selectivity of reaction favorably. It is also a drawback that, in extracting high-boiling impurities from the column bottom, catalysts, such as ferric chloride, generally used and present in the reaction mixture flow out and make it difficult to maintain the catalyst concentration in the reaction mixture at an adequate level. Furthermore, it is necessary to treat the iron-containing discharge liquid.
As a method of efficient utilization of the heat of reaction which is other than the reaction-distillation method, Japanese Patent Publication Kokai No. 74624/83 proposes a liquid circulation technique which comprises leading the liquid reaction medium in the reactor to a heat exchanger so that the sensible heat possessed by the medium can be utilized efficiently. In sensible heat utilization, a larger quantity of liquid has to be circulated as compared with the case where the latent heat of condensation of a vapor is utilized, thus the power cost for liquid circulation is increased. In addition, the cost of equipment is increased because of necessity of a larger heat exchanger due to a smaller heat transfer coefficient as compared with the heat transfer upon condensation or, in using the liquid as the heat source for a distillation column reboiler, unfavorable operations are required, for instance, operation to increase the temperature difference in the heat exchanger by operating the distillation column under reduced pressure to thereby lower the distillation temperature.
For keeping a favorable selectivity of reaction in the above high-temperature process, it is effective to use ethylene in excess relative to chlorine, as proposed in British Patent No. 1,184,576. When ethylene is used in excess, unreacted ethylene is discharged. Therefore, for securing a high raw-material-based yield, it is important to recover the unreacted ethylene discharged. On the other hand, the chlorine used in this process is mostly produced on a commercial scale by electrolysis of sodium chloride. The chlorine produced by mercury process contains about 0.2 to 0.5% of oxygen, and the chlorine produced by diaphragm process contains about 1 to 2% or more of oxygen. As described in Japanese Patent Publication Kokai No. 177928/83, oxygen is effective for maintaining the selectivity of the reaction on a good level and, for producing this effect, the oxygen contained in the chlorine or an additional quantity of oxygen can be utilized. The oxygen thus supplied to the reactor goes with and is contained in the discharge gas together with the unreacted ethylene and, therefore, it is important to take into consideration the risk that the discharge gas might form an explosive mixture composition.
Among the known methods of recovering the unreacted ethylene, the method disclosed in British Patent No. 1,184,576 comprises cooling the reaction medium vapor generated by the heat of reaction in a high-temperature process, separating the resulting EDC by condensation and supplying the uncondensed gas to a second reactor to thereby attain recovery of the unreacted ethylene. For this method, it is described that an inert gas is added to the uncondensed gas, and accordingly formation of explosive mixture compositions can be avoided. However, when an inert gas such as nitrogen is added to the unreacted ethylene discharge, the concentration of ethylene to be supplied to the second reactor is lowered by dilution with the inert gas and, as a result, ethylene absorption becomes difficult.
As an alternative, Japanese Patent Publication Kokai No. 57906/73 (Societa Italiana) proposes a method comprising carrying out the reaction in two steps by supplying at most 88% of ethylene to a second reactor in the unreacted state. Since the ethylene content in the discharge is high, formation of an explosive mixture can be avoided. When the load onto the second reactor is great, however, the above method is disadvantageous in that the heat recovery for efficient utilization of the heat of reaction as generated by the high-temperature reaction in the first reactor is decreased.
It is one of the problems encountered by the high-temperature method that the selectivity and the yield of the desired EDC are decreased, because by-products, including 1,1,2-trichloroethane, are formed in increased amounts as compared with the low-temperature method.
As methods proposed for inhibiting a side reaction which gives 1,1,2-trichloroethane in the high-temperature process, there are mentioned, for instance, a two-step reaction method disclosed in Japanese Patent Publication Kokai No. 57906/73 wherein the high-temperature reaction conducted using a large excess of ethylene and the low-temperature reaction for converting the excess ethylene are combined, a method of inhibiting side reactions chemically using an additive, such as a cresol (Japanese Patent Publication Kokai No. 40620/81), benzene or the like (Japanese Patent Publication Kokai No. 50203/83), or an amine (Japanese Patent Publication Kokai No. 104636/83).
A primary object of the present invention is to provide a method for producing EDC which is higher in heat utilization efficiency than the conventional reaction-distillation method and has no problems in selectivity of reaction and quality of product as encountered by the reaction-distillation technique and which is more efficient in recovery and utilization of reaction heat than the liquid circulation method.
The above and other objects of the present invention will become apparent from the description hereinafter.